When functioning properly, a heart maintains its own rhythm and is capable of pumping a sufficient amount of blood throughout a subject's circulatory system. However, some subjects may have cardiac arrhythmia. Generally, cardiac arrhythmia is a condition or group of conditions characterized by an irregular cardiac rhythm. In certain examples, cardiac arrhythmia can result in a diminished blood circulation throughout the body.
A cardiac arrhythmia can be treated using a cardiac rhythm management system. A cardiac rhythm management system can include an implantable or external system or device, such as a defibrillator, configured to deliver therapy, such as an electric stimulus, to the heart. Generally, a defibrillator can be used to deliver an electric stimulus, typically referred to as a defibrillation countershock or shock. The defibrillation countershock can interrupt an abnormal heart rhythm, allowing the heart to reestablish normal rhythm.
One problem faced by a cardiac rhythm management system is the determination of a threshold energy required, for a particular defibrillation shock waveform, to reliably convert an abnormal heart rhythm to normal heart rhythm. Ventricular fibrillation and atrial fibrillation are probabilistic phenomena that generally observe a dose-response relationship with respect to shock strength. The ventricular defibrillation threshold is the smallest amount of energy that can be delivered to the heart to reliably revert ventricular fibrillation or fast ventricular tachycardia to normal rhythm. Similarly, the atrial defibrillation threshold is the threshold amount of energy that will reliably terminate an atrial fibrillation. The defibrillation thresholds can vary from subject to subject, and may even vary within a subject depending on the placement of a lead or an electrode used to deliver the energy or depending on the subject's condition.
One technique for determining a defibrillation threshold includes inducing a targeted tachyarrhythmia (e.g., ventricular fibrillation), and then applying one or more than one shock of varying magnitude to determine the energy needed to convert the arrhythmia to normal heart rhythm. However, this technique requires imposing the risks and discomfort associated with both the arrhythmia and the defibrillation. Electric energy delivered to the heart has the potential to both cause myocardial injury or pain. As a result, anesthesia is generally required, adding an additional logistic barrier for implementation. Moreover, if defibrillation thresholds are being obtained in order to assist a physician in determining an optimal lead placement, these disadvantages are compounded as the procedure is repeated for different potential lead placements.
Another technique for determining the defibrillation threshold, referred to as the “upper limit of vulnerability” technique, includes shocking a subject that is in a state of normal heart rhythm during a vulnerable period of the cardiac cycle. The vulnerable period is generally a period when the heart tissue is undergoing repolarization, e.g., a R-wave period. Typically, one or more than one shock of varying magnitude is applied until fibrillation is induced. After such fibrillation is induced, the subject must again be shocked in order to interrupt the arrhythmia and reestablish normal heart rhythm. In this technique, the corresponding fibrillation-inducing shock magnitude is related, through a theoretical model, to a defibrillation threshold energy. The upper limit of vulnerability technique also suffers from imposing the risks and discomfort associated with both the arrhythmia and the defibrillation shock.
Moreover, because of the discomfort associated with the fibrillation and shocks, the subject is typically sedated under general anesthesia, which itself imposes additional risk and increased cost. For these and other reasons, the present inventor has recognized a need to estimate a defibrillation threshold without relying on a defibrillation countershock to induce or terminate an actual arrhythmia.